The instant invention relates to chemical vapor deposition (CVD) apparatus and methods, and more particularly to a parylene deposition apparatus including a quartz-crystal thickness/rate controller.
Parylene is a general term used to describe a class of poly-p-xylylenes which are derived from a dimer having the structure ##STR1## wherein X is typically a hydrogen, or a halogen. The most commonly used forms of parylene dimers include the following: ##STR2##
Parylene coatings are obtained from these parylene dimers by means of a well-known vapor deposition process in which the dimer is vaporized, pyrolized, i.e. cleaved into a monomer vapor form, and fed to a deposition chamber wherein the monomer molecules deposit and polymerize onto a substrate disposed within the deposition chamber. The process occurs according to the following reaction: ##STR3##
Due to their ability to provide thin films and conform to substrates of varied geometric shapes, parylene polymers are ideally suited for use as a conformal external coating in a wide variety of fields, such as for example, in the electronics, automotive, and medical industries.
Octafluoro-[2,2]paracyclophane (Parylene AF4 dimer) is a fluorine substituted version of the above-noted parylenes and has the structure: ##STR4## It is known that parylene coatings which are derived from the AF4 dimer by the vapor deposition process have a very high melting temperature (about 500.degree. C.) and a very low dielectric constant (about 2.3). These characteristics make Parylene AF4 ideally suited for many high temperature applications, including electronic applications, and potentially as an inter-layer dielectric material in the production of semiconductor chips.
The existing parylene coating systems as used with Parylene C, D, and N, typically include a chamber system comprising a vaporization chamber, a pyrolysis chamber coupled to the vaporization chamber wherein the vaporized dimer is cleaved into monomer form, and a deposition chamber coupled to the pyrolysis chamber in which the monomer vapor deposits onto a substrate and polymerizes into a thin film coating. The known coating systems further include a vacuum system coupled to the chambers for creating sub-atmospheric pressure conditions throughout the chamber system. While the existing parylene deposition systems are highly effective in depositing parylene C, D, and N, there are unique characteristics of the Parylene AF4 molecule which prevent the existing parylene coating systems from providing sufficient deposition control, uniformity of layer thickness, material efficiency, and speed of coating to be compatible with existing semiconductor chip manufacturing technologies, semiconductor chip cost structures, and semiconductor chip manufacturing time constraints. Accordingly, there is currently presented a need for a parylene deposition system including means for accurately controlling the rate of deposition of parylene AF4 onto semiconductor wafers.
The instant invention provides a parylene deposition system comprising a vaporization chamber, a pyrolysis chamber, a deposition chamber, and a vacuum assembly for creating sub-atmospheric conditions within the chambers. More specifically, the vaporization chamber comprises a cylindrical housing having an inlet end and an outlet end. The inlet end of the housing is provided with a hinged door to permit selective access to the interior of the housing for the placement of Parylene AF4 dimer into the vaporization chamber. Located inside the vaporization chamber is a dimer heating device which is effective for vaporization of the dimer. The powdered AF4 dimer is received in a removable dimer crucible, which is in turn received into a heat transfer receptacle. The heat transfer receptacle is preferably heated by a plurality of electric heating elements disposed around the receptacle in intimate thermal contact with the receptacle. The electric heating elements are operative for quickly heating the receptacle to a temperature which is suitable for the vaporization of the dimer. The dimer heating device further includes a cooling assembly for quickly cooling the heat transfer receptacle to a temperature below the vaporization temperature of the dimer. The cooling assembly comprises a heat exchange coil disposed in intimate thermal contact with the heat transfer receptacle and a heat exchange pump for circulating a cooler fluid through the heat exchange coil. The cooling assembly can be utilized to quickly cool the dimer to quench vaporization, i.e. reduce the rate of vaporization. A temperature sensor is operative for monitoring the temperature of the heat transfer receptacle.
The outlet end of the vaporization chamber is coupled to an inlet opening of the pyrolysis chamber by a pipe. The pipe extending between the vaporization chamber and the pyrolysis chamber preferably includes a throttle valve for controlling the flow of vaporized dimer into the pyrolysis chamber.
The parylene deposition apparatus further comprises an electronic controller operative for electronically controlling all aspects of the deposition process, including temperature and pressure regulation, and still further includes a quartz-crystal deposition rate control system including a quartz crystal assembly disposed within the deposition chamber. In operation, the parylene monomer vapor deposits onto the surface of the crystal varying the vibration frequency of the crystal. The rate of change in frequency can be directly correlated with rate of deposition of the parylene monomer onto the crystal, and thus onto the substrate to be coated. The crystal is electrically connected to an oscillator for measuring the frequency of the crystal which varies with the thickness of the coating on the crystal. The oscillator is connected to the micro-controller which measures the frequency changes of the crystal and continuously calculates the rate of deposition of the parylene monomer vapor. Furthermore, the microprocessor is connected to a set point comparator which compares the calculated deposition rate to a set or desired deposition rate. The microprocessor then controls operation of the heating and cooling elements of the dimer heat transfer receptacle to increase or decrease the temperature of the receptacle and thereby control vaporization. The microprocess also controls opening and closing of the throttle valve to provide more or less parylene dimer to the system.
Accordingly, among the objects of the instant invention are: the provision of a parylene deposition apparatus effective for quick and efficient deposition of Parylene AF4 onto silicon wafers in the production of semiconductor chips; the provision of a parylene deposition apparatus including a heated and cooled dimer crucible for fast and efficient vaporization of the dimer material; the provision of a deposition rate control system for measuring and controlling the rate of parylene AF4 deposition; and the provision of further means for the fast, efficient, and cost effective deposition of Parylene Af4 onto the surface of a silicon wafer.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.